FIG. 1 illustrates a network structure of an Evolved Universal Terrestrial Radio Access Network (E-UTRAN), which is a mobile communication system to which the related art and the present invention are applied. The E-UTRAN has been evolved from the conventional UTRAN and its basic standardization process is currently underway in 3GPP. The E-UTRAN system is also referred to as a Long Term Evolution (LTE) system.
The E-UTRAN includes base stations which will each be referred to as an eNode B or an eNB for short. The eNBs are connected through X2 interfaces. Each eNB is connected to User Equipments (UEs) and is connected to an Evolved Packet Core (EPC) through an S1 interface.
Radio interface protocol layers between UEs and the network can be divided into a L1 layer (first layer), a L2 layer (second layer), and a L3 layer (third layer) based on the lower three layers of the Open System Interconnection (OSI) reference model widely known in communication systems. A physical layer included in the first layer provides an information transfer service using a physical channel. A Radio Resource Control (RRC) layer located at the third layer controls radio resources between UEs and the network. To accomplish this, the RRC layer exchanges RRC messages between UEs and the network.
FIG. 2 illustrates a radio interface protocol structure between a UE and a UTRAN based on the 3GPP radio access network standard. The radio interface protocol of FIG. 2 is divided horizontally into a physical layer, a data link layer, and a network layer and is divided vertically into a user plane for data/information transmission and a control plane for signaling. The protocol layers of FIG. 2 can be divided into a L1 layer (first layer), a L2 layer (second layer), and a L3 layer (third layer) based on the lower three layers of the Open System Interconnection (OSI) reference model widely known in communication systems.
A physical layer, which is the first layer, provides an information transfer service to an upper layer using a physical channel. The physical layer is connected to a Medium Access Control (MAC) layer above the physical layer through a transport channel. Data is transferred between the MAC layer and the physical layer through the transport channel. Data transfer between different physical layers, specifically between the respective physical layers of transmitting and receiving ends, is performed through the physical channel. The physical channel is modulated according to the Orthogonal Frequency Division Multiplexing (OFDM) method, using time and frequencies as radio resources.
The MAC layer, which is the second layer, provides a service to a Radio Link Control (RLC) layer above the MAC layer through a logical channel. The RLC layer of the second layer supports reliable data transfer. Functions of the RLC layer may be embodied in a function block in the MAC. In this case, the RLC layer may not be provided. A PDCP layer of the second layer performs a header compression function to reduce the size of each IP packet header containing relatively large, unnecessary control information in order to efficiently transmit IP packets such as IPv4 or IPv6 packets in a radio interval with a small bandwidth.
A Radio Resource Control (RRC) layer located at the top of the third layer is defined only in the control plane and is responsible for control of logical, transport, and physical channels in association with configuration, re-configuration, and release of radio bearers (RBs). The RB is a service that the second layer provides for data communication between the UE and the UTRAN. The UE is in a connected mode if there is an RRC connection between the RRC layer of the radio network and the RRC layer of the UE. Otherwise, the UE is in an RRC idle mode.
A Non-Access Stratum (NAS) layer located above the RRC layer performs functions such as session management and mobility management.
One cell included in the eNB is set to provide a bandwidth such as 1.25, 2.5, 5, 10, or 20 MHz to provide a downlink or uplink transmission service to UEs. Here, different cells may be set to provide different bandwidths.
Downlink channels used to transmit data from the network to the UE include a Broadcast Channel (BCH) used to transmit system information, a Paging Channel (PCH) used to transmit paging messages, and a downlink Shared Channel (SCH) used to transmit user traffic or control messages. Control messages or traffic of a downlink multicast or broadcast service may be transmitted through a downlink SCH and may also be transmitted through different downlink multicast channels (MCHs). Uplink channels used to transmit data from the UE to the network include a Random Access Channel (RACH) used to transmit initial control messages and an uplink SCH used to transmit user traffic or control messages.
Examples of a logical channel, which is located above the transport channel and is mapped to the transport channel, include a Broadcast Control Channel (BCCH), a Paging Control Channel (PCCH), a Common Control Channel (CCCH), a Multicast Control Channel (MCCH), and a Multicast Traffic Channel (MTCH).
FIG. 3 illustrates conventional control channel transmission. The physical channel includes a number of subframes on the time domain and a number of subcarriers on the frequency domain. One subframe includes a plurality of resource blocks, each of which includes a plurality of symbols and a plurality of subcarriers. In each subframe, specific subcarriers of specific symbols (for example, the first symbol) of the subframe can be used for a Physical Downlink Control Channel (PDCCH), i.e., an L1/L2 control channel. One subframe corresponds to 0.5 ms and a Transmission Time Interval (TTI), which is a unit data transmission time, is 1 ms corresponding to two subframes.
Reference will now be made to a Multimedia Broadcast/Multicast Service (MBMS). The MBMS provides a streaming or background service to a plurality of UEs using a downlink only MBMS bearer service.
The MBMS is divided into a broadcast mode and a multicast mode. The MBMS broadcast mode is a service of transmitting multimedia data to all users in a broadcast area where a broadcast service is available. The MBMS multicast mode is a service of transmitting multimedia data to a specific user group in a multicast area where a multicast service is available.
Conventional paging channel transmission will now be described with reference to FIG. 4. When receiving a paging message, the UE can perform Discontinuous Reception (DRX) in order to reduce power consumption. To accomplish this, the network constructs a number of paging occasions in each period of time, which is referred to as a paging DRX cycle, and a specific UE receives a specific paging occasion to obtain a paging message. The UE receives no paging channel at any time other than the specific paging occasion. One paging occasion corresponds to one TTI.
The following is a description of an MBMS dual layer service. The network may have one or more frequency bands in one area. However, if the respective ranges of the cells of two frequency bands are identical, it is inefficient to transmit the same MBMS service over each of the frequencies. To solve this, if a plurality of frequency bands are available, the eNB allocates a specific one of the frequency bands exclusively to the MBMS to provide an MBMS service over the specific frequency band and to provide other services of each UE (for example, a unicast service) over the other frequencies.
That is, when a number of frequency bands are available, in order to increase the efficiency of the provision of the MBMS service, the eNB sets one of the frequency bands as an MBMS dedicated layer (or frequency band) so that only the MBMS service is allowed to be provided over the MBMS dedicated layer. In other words, the eNB does not provide a unicast service for a specific UE only over the MBMS dedicated layer to efficiently provide the MBMS service. Accordingly, channels not associated with the MBMS (for example, channels such as an RACH and a UL-SCH) are not used in the MBMS dedicated layer. Any paging message is also not transmitted over the MBMS dedicated layer.
If a number of frequency bands are available in one area and the MBMS service is provided over only one of the frequency bands, it is necessary for UEs in the area to switch to the frequency band in order to receive the MBMS service. However, each UE may desire to receive the unicast service at the same time. When receiving a unicast paging message, the UE must be able to immediately receive the paging message.
Regardless of whether or not a MBMS service is provided over a frequency band set to provide MBMS services, each UE can receive a paging message transmitted over another frequency band at any time if the UE is capable of receiving more than one frequency band at the same time.
However, while receiving the MBMS service, a single-receiver UE cannot normally receive a paging message transmitted over another frequency band if the UE is capable of receiving only one frequency band.